At 10 minutes for 10,000 importations each unit is taking 6ms. 10 - 15 minutes ain't bad.

Let me tell you what I understand of what you're doing from your description.

Many Users, each User has unique 10K item task list. Load task list into array, distribute array into 3 ea 10K item dictionarys. Compare Array Items to first dictionary and process or use second dictionary to processs. Discard third dictionary. Send something to MSProject.

Can you explain your complex and lengthy process with a bit more detail?

A good explanation might take two screens of text, compared to the one brief paragraph you now have.